Harry,.
It's all about compromises. Lower ISO - better detail (less digital noise (film-grain like artifact)), but more light needed (wider opened diaphragm (shallow focus area), or longer shutter speed(moving object got blurred)).
Higher ISO - shorter shutter speed, smaller diaphragm opening, but noisier detail.http://www.stan-pustylnik.smugmug.com..
ISO regulates sensor sensitivity. If you are using a fast shutter speed to stop a moving object in dim light, you may need to increase the ISO. On my 350d, I routinely increase the ISO to 800 or even more when the light and shutter speed demands require it. A higher ISO increases "noise," but not badly, and it can be taken out with PP..
Jerryhttp://jchoate.zenfolio.com/..
Apart from the noise issues: lower ISOs usually result in cleaner, less noisy images, there are other reasons way you may want to use a low sensitivity..
As has already been mentioned, you may actually want something blurred. Good examples of this would be waterfalls. These are often shot at slow shutter speeds to capture the flow of the water. Sometimes darkening filters (called neutral density filters) are used to force even slower speeds, when the ISO can't be lowered enough..
Another example would be when shooting in extremely bright conditions (say bright sunshine at the beach or in snow), when a shallow depth of field is wanted. To get a shallow depth of field, a wide aperture is needed, and using a wide aperture bumps up the shutter speed. This is generally fine with professional dSLRs which often have shutter speeds of 1/8000 s or more., but for many cameras the maximum shutter speed could be exceeded without lower ISOs (and/or ND filters)..
Brian A...
I call the principles of exposure the "LAST" principle, for Light, Aperture, Sensitivity, Time. All four aspects affect the others in a tradeoff, but also control something of their own (balance, depth, grain, motion)..
Http://halley.cc/photo/last.html.
If you want to control the motion (say, make a milky smooth waterfall effect instead of a chunky one), you want to set your Time and trade for it with the others. If you can't or don't want to trade with less light nor smaller aperture, then you have to trade with less sensitivity. If your camera won't let you use anything below ISO200, you will have to remember to bring a Neutral Density filter to deal a fair exposure with less light instead of less sensitivity..
[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..
# The general rule for optimum QUALITY of image...is to use the LOWEST ISO rating you can *get away with* for ease of shooting ...(in the light strength conditions prevailing)..
Or, alternatively......
# The general rule for optimum EASE of shooting the image... is to use the HIGHEST ISO you can *get away with* in quality terms ...(in the light strength conditions prevailing)..
Which of these two rules to apply, or which compromise between the two you settle on, is something YOU will have to decide each and every time of shooting. The other posters have given you some idea of the considerations: be pleased you have a choice, and can change ISO between shots when necessary. Regards,Baz..
Thanks for all your answers. After again reading your answers, it is quite difficult to "choose" the right ISO for certain condition. Only experience can tell ....
Well, I am an amateur anyway (really amateur!) so I should use "auto-ISO" when it is available .
I think, only professional could really say "hey, in this light condition, you must use ISO x"..
PS: Is there any medicine that I can eat to make me a pro photographer? j/k..